This invention relates to the biological purification of waste waters by methods comprising treatment with either anaerobic or aerobic microorganisms. More particularly it relates to the treatment of waste waters containing formaldehyde, in which processes the formaldehyde-containing waste water is introduced into a suitable biological digestor along with a second, nitrogenous stream which contains any of several types of nitrogen compounds as will be explained more fully hereinbelow. Especially, however, the invention relates to the treatment of a formaldehyde-containing waste water in a system within which a nitrogen compound, as exemplified by ammonia, is introduced into the digestor along with the waste water for the purpose of supplying nitrogen nutrient for the contained biomass.
Biological waste water treatment systems comprise three main types: (a) aerobic systems, (b) anaerobic systems, and (c) combination systems in which a first anaerobic stage is followed by a second aerobic stage. The aerobic systems are inherently, to some degree at least, back-mixed by the turbulence caused by the air which is essential to the process and which is commonly sparged into the bottom of the digestor vessel. The anaerobic systems include both back-mixed digestors and others, especially including certain types of anaerobic filter, which approach once-through operating conditions. An especially useful type of anaerobic filter operates with a recycle of the effluent liquid back to the filter inlet whereby a back-mixed environment obtains in the anaerobic filter digestor. Such a back-mixed recycling anaerobic filter is described in Belgian Pat. No. 828916 by E. R. Witt et al assigned to Celanese Corporation.
Until recently, the waste waters which were treated by biological digestion methods have been from natural, as distinguished from industrial, sources, as exemplified by municipal sewage. It will be understood that much of the published technology in the field has spoken of the wastes from food processing plants and slaughter houses, for example, as being industrial wastes, but it will be seen that these are essentially natural materials even though they may in some cases be of extremely high concentration so as to present some difficulty in processing. Effluents from the synthetic chemical industry, which are not of natural origin, have not been treated by biological methods until quite recently, and even now the biological treatment of such materials is in its infancy. Only recently, for example, has any attention been paid to attempting the biological treatment of such compounds as formaldehyde and phenol, both of which have been thought of as biocidal or biostatic.
While it is now coming to be realized that formaldehyde in particular is in fact amenable to biological digestion, it has not as yet been realized that its processing requires any special considerations other than the (arguably at least) obvious one of avoiding such excessively high concentrations as to overload the biomass. There is also the now generally-understood consideration that formaldehyde and other synthetic chemicals require, as distinguished from substances which are of natural origin, the incorporation of biomass nutrients, especially nitrogen and phosphorus, into the waste water which is to be treated. Natural materials, such as sewage, food-processing wastes, and slaughter house wastes, usually include sufficient biomass nutrient moiety that supplemental nutrients are not essential.
Thus, the biological digestion of waste water containing formaldehyde usually, if not always, calls for incorporating biomass nutrients, including specifically a nitrogen source, into the waste water which is to be treated. Such nitrogenous nutrients can comprise ammonia (which is especially advantageous because of its low cost and ease of distribution) or, alternatively, its obvious equivalents, e.g., ammonium hydroxide, ammonium salts, urea, etc. Such supplemental nutrients, when they are employed, have normally, in the prior art, been conveniently incorporated into the waste water before it is introduced into the biological digestor. There are several reasons for this. For example, admixing of the nutrient into the waste water is facilitated when this is done as far as possible upstream from the digestor inlet. That is, in passing through pumps, bends in the piping, etc. together, the waste water and the nutrients are thoroughly mixed without the need for special mixing equipment. Also, biological digestion systems frequently include a holding pond or equivalent surge tank ahead of the biological digestor itself for the purpose of smoothing out undesirable surges in organic loading of the digestion system. Such holding ponds or tanks can be made to serve a gauging function, and here again there are practical advantages in adding a measured quantity of nutrient into a measured quantity of waste water as it is admitted into these holding vessels. In summary, practical considerations lead one of normal skill in the art to mix the nutrient, including specifically the nitrogenous portion of the nutrient, into the waste water well ahead of the point at which it enters the biological digestor. These same considerations apply whether the digestion system is aerobic, anaerobic, or a combination of the two.
It has recently been observed, however, that in treating waste waters containing formaldehyde there have been times when the chemical oxygen demand of the digestor outfall liquid began to increase to an undesirably or unacceptably high level even though the biomass remained in good condition, when there had been no surge in organic concentration in the waste water being fed, and when there had been no known admixture of unacceptable levels of heavy metals or other substances adverse to biomass activity into the system. The present invention is directed to means which have been discovered for coping with this condition.
It is, accordingly, an object of the present invention to provide an improved method for carrying out the biological digestion of waste waters containing formaldehyde. It is a further object to provide a method for alleviating the undesirable conditions sometimes obtaining when the oxygen demand of the digestor outfall increases in digestors processing formaldehyde-containing waste waters. It is a particular object to improve the treatment efficiency in biological digestion systems into which there are introduced both a formaldehyde-containing waste water and also a stream comprising a nitrogenous compound as will be more particularly explained hereinbelow. Other objects will be apparent from the following detailed description and examples.